Because the failure of superficial endothelial injuries to heal appears to play a role in the development of atherosclerosis and vasospasm, factors which could speed the healing are of interest for these processes as well as for the potential to enhance angiogenesis and thus prevent myocardial infarction. In this study, radioiodine labelled endothelial cells growth factor was prepared using endothelial cell growth factor (purified from bovine brain by heparin sepharose chromatography) and lactoperoxidase. The radiolabelled growth factor was shown to be active as an endothelial cell mitogen. In anesthetized rats, one carotid was denuded using an embolectomy balloon and the denudation was confirmed by Evans blue staining as well as by scanning electron microscopy. The animals were then injected intravenously with the radiolabel growth factor with or without heparin. Heparin was found to increase the binding to the intact carotid. The denude carotid bound 8 times as much ECGF as normal carotid, presumably reflecting binding to the extracellular matrix. This was supported by the fact that premixing of the growth factor with heparin decreased the binding to the matrix, although it did not return it to normal. These were the first studies of the binding of endothelial cell growth factor in vivo and they lend support to the idea that such growth factors may be useful to promote endothelial healing, which might be helpful for wound healing, seeding of vascular prosthesis, and to promote angiogenesis.